Thanks for looking, Ivo. I'll try to post an additional base picture tomorrow.
The reason I thought it was more like my Czech pieces is because it's a bird-bath style base, where the outer contour of the base looks matt (just like most of my Rosice and Hermanova pieces). On many of my German pieces, that bird bath style is either not there, or if there is a sort of rim around the base, it's polished clear. Also, the rim is polished in such a way that it 'slopes' on two sides, which looks a bit similar to the rim finish of my Jurnikl jardiniere (though that one slopes differently, more to one side) - (at times like these I feel the lack of the right glass vocabulary to properly describe it, sigh).
Anyway, I agree that sort of similarity is not enough to be sure about the origin.
So far, I've checked 6 or 7 Nachtmann pieces that I could dig up easily enough and they don't have that style of base. None of my German pieces use that, though I seem to remember a couple of glasworks in Poland and East Germany did use the bird-bath style?
French though would be another possibility - I don't have any Sevres pieces that I know of to compare (my Crystal d'Arques pieces have different style bases).
And I'm too much of a nitwit to compare the clarity of crystal/glass used to that of certain companies. I can't even tell if it's glass or crystal

Astrid